Tobacco Documents Online

1 NOTES ON "AGE RELATED CHANGES IN AGE OF STARTING TO SMOKE" * This paper published in the Journal of Clinical Epidemiology addresses itself to these currently common beliefs: There is an increase in individuals in the younger age cohorts (early teens or younger) who take up smoking. The shift of smoking to earlier ages is especially prominent among young women. The prevalence of smoking among women has actually increased. The analysis shows that none of the three beliefs are justified. The first two propositions a~e due to two separate errors of calculations and the third is simply not true. One of the errors (leading to false conclusions in support of 1 and 2 above) was made by Garfinkel and Stellman of the American cancer Society and also, in another paper, by Harris (well known for his work on smoking and health). That error is of a type that has been made before in the history of epidemiology (usually to the embarrassment of the perpetrator). Garfinkel and Stellman and also Harris compute the average age for starting to smoke separately for younger and younger birth cohorts (i.e., groups of subjects classified according to when they were born which in turn determines their current age) They observed a decreasing average of starting to smoke for successive birth cohorts of decreasing age from which they deduce the existence of a trend by which the typical age at which people start to smoke is getting progressively younger. The error in their method is to ignore the fact that the average age at which any event takes place, computed for different age groups, actually depends on the age of the cohort. So, as a simple example, the average age of starting to smoke among high school students will always be less than the average age of starting to smoke among college students, simply because college students tend to be older than high school students. Some college students will have taken up smoking after age 19; few if any high school student can have done this because almost all are younger than 19. This "pulls" the average up for college students. - *Weinkam, J., Sterling, T. "Age Related Changes in Age of Starting to Smoke". Journal of Clinical Epidemiology 43(2):451-461, 1990.

A different error (with respect to 1 and 2 above) is committed by the 1989 Surgeon General's Report. This Report also bases its conclusions about younger and younger people taking up smoking on the average age of starting to smoke. However this Report corrects the estimates of average age of starting to smoke for age dependency. The Report observes a progressive decrease in the average age of starting to smoke with younger years for white females (but not for white males or for blacks). However, there are two reasons why such an observation might occur: It might be true that more younger white females are starting to smoke, or It might be true that fewer older females take up smoking. By comparing two separate age groups, using data from the National Center for Health Statistics, the study shows that the reason for the apparent decline in the average age of starting to smoke for white females is that proportionately fewer older females (i.e., late teens) start smoking. With respect to the third proposition, between 1970 and 1980 the prevalence of smoking among white males younger than 24 years of age decreased by 6% and that of white females for the same age group decreased by 4.7%. For blacks the decreases were 12.4% and 7.7% respectively. (However, the estimates for blacks are less stable than those for whites because they are based on a relatively small sample.) The available data show that all three common propositions about the age of starting to smoke and the prevalence of smoking among young people are false. The Weinkam/Sterling paper was carefully reviewed and satisfied the critical examination of a number of leading epidemiologists and statisticians. Also, the study's conclusion concerning the three propositions commonly encountered are easily verifiable from data available from the National Center for Health Statistics and from other sources. This paper by Weinkam and Sterling furnishes the most clear cut demonstration yet on how exaggerated and erroneous conclusions are made about smoking by sources of great authority and influence in matters of public health. Its dissemination might help put the status of smoking among women and the young in a more rational perspective.